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New insights into the Kawah Ijen hydrothermal system
from geophysical data
CORENTIN CAUDRON1,2,3*, GUILLAUME MAURI4,5, GLYN WILLIAMS-JONES5,
THOMAS LECOCQ2, DEVY KAMIL SYAHBANA6, RAPHAEL DE PLAEN7,
LOIC PEIFFER8, ALAIN BERNARD3 & GINETTE SARACCO9
1
Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Avenue,
Block N2-01a-15, Singapore 639798
2
Royal Observatory of Belgium, Seismology Section, 3 avenue Circulaire,
1180 Uccle, Belgium
3
Department of Earth and Environmental Sciences, Université Libre de Bruxelles, 50 Avenue
9 Roosevelt, 1050 Brussels, Belgium
4
Center for Hydrogeology and Geothermics, University of Neuchtel, Neuchtel, Switzerland
5
Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby,
British Columbia, V5A 1S6, Canada
6
Centre for Volcanology and Geological Hazard Mitigation, Geological Agency, Ministry
of Energy and Mineral Resources, Jalan Diponegoro 57, Bandung 40122, Indonesia
7
University of Luxembourg, Faculté des Sciences, de la Technologie et de la Communication,
6 rue Richard Coudenhove-Kalergi, L-1359 Luxembourg
8
Instituto de Energı́as Renovables, Universidad Nacional Autónoma de México, Privada
Xochicalco s/n, Centro, 62580 Temixco, Morelos, Mexico
9
CNRS-UMR7330, Centre Europeen de Recherche et d’Enseignement en Geosciences de
l’Environnement (CEREGE), Equipe Modelisation, Aix-Marseille Université (AMU),
Europole de l’Arbois, BP 80, 13545 Aix-en-Provence cedex 4, France
*Corresponding author (e-mail: corentin.caudron@gmail.com)
Abstract: The magmatic– hydrothermal system of Kawah Ijen volcano is one of the most exotic
on Earth, featuring the largest acidic lake on the planet, a hyper-acidic river and a passively degassing silicic dome. While previous studies have mostly described this unique system from a geochemical perspective, to date there has been no comprehensive geophysical investigation of the
system. In our study, we surveyed the lake using a thermocouple, a thermal camera, an echo sounder and CO2 sensors. Furthermore, we gained insights into the hydrogeological structures by combining self-potential surveys with ground and water temperatures. Our results show that the
hydrothermal system is self-sealed within the upper edifice and releases pressurized gas only
through the active crater. We also show that the extensive hydrological system is formed by not
one but three aquifers: a south aquifer that seems to be completely isolated, a west aquifer that sustains the acidic upper springs, and an east aquifer that is the main source of fresh water for the lake.
In contrast with previous research, we emphasize the heterogeneity of the acidic lake, illustrated by
intense subaqueous degassing. These findings provide new insights into this unique, hazardous
hydrothermal system, which may eventually improve the existing monitoring system.
Kawah Ijen volcano (Java Island, Indonesia) (Fig. 1)
hosts a very active hydrothermal system that
consists of the largest hyper-acidic lake on Earth
(current volume c. 30 million m3, pH 0 and temperature .308C), a hyper-acidic river that flows
down the western flank of the edifice, and an
actively degassing high-temperature silicic dome
(.2008C) (van Hinsberg et al. 2010). Hydrothermal
activity within an active crater hosting a lake
presents a significant hazard for the surrounding
From: Ohba, T., Capaccioni, B. & Caudron, C. (eds) Geochemistry and Geophysics of Active Volcanic Lakes.
Geological Society, London, Special Publications, 437, http://doi.org/10.1144/SP437.4
# 2016 The Author(s). Published by The Geological Society of London. All rights reserved.
For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
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C. CAUDRON ET AL.
Fig. 1. Map of Indonesia, showing the location of Kawah Ijen (East Java). The main panel shows the view from the
west of Kawah Ijen volcano and its main features. Solid (green) line, Self-Potential (SP) profiles; three-pronged
symbols, CO2 measurements on the lake; black squares, reference stations; open circles, houses/shelters; dashed
(cyan) line, Banyu Pahit river; shaded (blue) surface, lake; The dotted (red) line indicates the echo sounding profile
used in Figure 4; diamonds, temperature sensor locations; three stars, springs. See online version for colours.
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KAWAH IJEN HYDROGEOLOGY
population and farming activities. Since the beginning of the twentieth century, a concrete dam on
the west side of Kawah Ijen’s crater has controlled
the lake overflow that could flood the surrounding
villages and fields (Caudron et al. 2015a). Further
west, the acid river passes through many coffee
plantations and other crops. Hydrothermal activity
weakens the volcanic edifice over time (Opfergelt
et al. 2006), increasing the risk posed by strong
phreato-magmatic or magmatic explosions, as well
as catastrophic release of the contaminated lake
water that might be externally triggered (e.g. by
an earthquake or lake drainage; Caudron et al.
2015a). Hence, a better knowledge of the hydrothermal system and its extent are required to improve
forecasting of such catastrophic events. The hydrothermal system has thus been studied for more than
a century, mostly by geochemists (Caron 1914;
Hartmann 1917; van Bemmelen 1941; Mueller
1957; Delmelle & Bernard 1994, 2000; Takano
et al. 2004; van Hinsberg et al. 2010, 2015; Palmer
et al. 2011). Delmelle et al. (2000) developed the
only existing volcano-hydrothermal model in which
high-temperature magmatic gases are absorbed
into shallow groundwater and produce a two-phase
vapour–liquid hydrothermal reservoir beneath the
crater. In this model, the condensing gases may
have equilibrated in the liquid –vapour hydrothermal region at c. 3508C. The liquid phase of the
hydrothermal reservoir consists of SO4 –Cl waters
that enter a convective cell through which the water
circulates. The summit SO4 –Cl reservoir flows laterally and mixes with meteoric-dominated water to
produce the spring discharges of the hyper-acidic
river. The model of Delmelle et al. (2000) is derived
from the chemical and isotopic characteristics of
the thermal water and fumarole discharges.
Geophysical studies have been conducted on volcanic lake surfaces and edifices to locate, discriminate and differentiate aquifers from each other
(e.g. Finizola et al. 2002; Lewicki et al. 2003;
Mauri et al. 2010), to image specific features at the
lake surface (Ramı́rez et al. 2013) and to image subaqueous degassing features (Caudron et al. 2012).
However, to the best of our knowledge these techniques have not been applied to the Kawah Ijen
environment. Our geophysical investigations were
conducted between July 2006 and August 2011.
During this time, volcanic activity did not exceed
the background values in seismic and volcanic
lake parameters (Global Volcanism Program 2007,
2009; Caudron et al. 2015a) and hence, was considered as normal (alert 1 on a scale of 1–4). A swarm of
distal volcano-tectonic earthquakes occurred on 21
May 2011 near the Kawah Ijen caldera, just prior
to the last geophysical survey, and may have triggered the subsequent unrest that started in October
2011 (Caudron et al. 2015b).
Instruments and methods
Temperature monitoring and survey
The lake was initially surveyed for thermal anomalies using a type-K thermocouple suspended from a
boat during the day and by thermal IR (Infrared,
FLIR camera P25 PAL model) imaging from the
crater rim at night. Based on the acquired information, a total of about 20 iButton temperature probes
(accuracy of c. 0.58C, resolution of c. 0.68C) were
placed at strategic locations to monitor the lake
over the last five years (Caudron et al. 2015a, b).
Four iButton probes were first embedded in a polyphenylene sulphide capsule with a silicone O-ring to
protect the sensors from acidity. However, the protection proved inadequate and all four probes were
lost. All other probes were therefore placed in sample bottles filled with neutral water. In addition, to
avoid any acidic water infiltration, each bottle was
wrapped with thick tape and weighted with an iron
rod. A Teflon or nylon rope was wrapped around
each bottle neck and protected by an additional
layer of tape. The bottles were then suspended in
the lake, while the rope was attached to a climbing
piton stabilized near the lake edge. Depths of the
bottles typically fluctuated between 1 and 5 m
depending on the lake level at a given time.
Several iButtons were also installed in the Banyu
Pahit River (,20 cm deep) at the upper spring sites
(Palmer et al. 2011) (number 1, Fig. 1).
The crater rim ground temperature was surveyed
every 20 m along the self-potential profiles with
a K-type chrome-aluminum probe (accuracy of
c. 0.28C). In order to characterize any atmospheric
influence, ground temperature was measured at
.20 cm depth where possible. Atmospheric temperature was also measured at each survey point.
CO2 and degassing surveys
In 2007 and 2008, we attempted to record CO2 concentrations every 20 m along the self-potential profiles (Fig. 1) with a CO2 meter (a Vaisala GM70 with
a GMP221 probe) with a concentration range up to
20% and an error of 0.02% CO2 + 2% of the reading
value. These attempts were unsuccessful because
the fine ash surface layer was too compact for gas
pumping and ash particles regularly blocked the
gas probe filter.
In 2010 and 2011, a miniaturized NDIR CO2 gas
analyser (Bernard et al. 2013) suspended from a
boat at a depth ,30 cm recorded high concentrations of free CO2 dissolved at shallow depths in
the lake waters. The dissolved CO2 concentrations
were almost constant throughout the lake area.
In June 2010 and July 2011, degassing areas
and lake bathymetry were also determined with a
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C. CAUDRON ET AL.
single-beam dual frequency (50–200 kHz) Simrad
echo sounder. When using a single-beam echo
sounder in volcanic lake environments, the backscattering strength (Sv, in dB), corresponding to
the concentrations of echoes in the water column,
is the most reliable parameter (Caudron et al.
2012) and is calculated by:
Is 1
Sv = 10 × log
Ii v
(1)
where the variables are defined as: v, pulse volume
(in m3); Ii, incident intensity (in dB); Is, scattered
intensity (in dB). The ratio [(Is/Ii) (1/v)] is also
termed the backscattering coefficient or sv (in m21).
Self-potential (SP)
To investigate the flow direction of groundwater and
characterize the type of flow, the self-potential (SP)
method was used (e.g. Corwin & Hoover 1979;
Lénat 2007; Jouniaux et al. 2009; Mauri et al.
2012). A detailed description of the method can be
found in the work of Lénat (2007), Jouniaux et al.
(2009) and Aizawa et al. (2008).
The self-potential survey was carried out every
year between 2006 and 2009 at the end of July and
the beginning of August, i.e. during the dry season.
Two SP profiles were surveyed every 20 m along the
two main access routes. One profile was along the
crater rim and the other to the south of the edifice
(Fig. 1). In 2007, an additional south –north radial
profile was created, going from the south crater
peak, near the seismic station, to Pondok, where it
branched to the north, ending at the Banyu Pahit
River, and to the west, ending at Paltuding (Fig. 1).
SP was measured with non-polarized copper/copper
sulphate electrodes, whose polarity was checked at
least twice a day. The measurements were recorded
with a high-impedance multimeter (c. 200 Mohm).
Each 20 m location was referenced with a handheld GPS and measurements were always taken in
a shallow hole (from 5 to 20 cm deep) to ensure sufficient ground moisture. The resistivity contact of
the electrode with the ground was always below
120 kohm, which gives a good level of confidence
in the quality of the electrical potential (mV) measured in each hole. To determine whether the SP values reflected natural generation rather than poor
contact between the electrode and moist ground,
each record of the electrical potential was also complemented by a record of the contact resistance
(kohm) between the electrode and the ground.
All self-potential profiles were closed in a loop
or directly connected to a natural water source.
Three different springs were used as 0 mV reference
points: the Banyu Pahit spring (numbers 2, 4, Fig. 1),
the Inner crater spring (number 5, Fig. 1) and the
Paltuding spring (number 6, Fig. 1). In 2007, the
Banyu Pahit River, where it crosses the road, was
also used as reference. The Kawah Ijen acid lake
was not used as a reference due to significant signs
of chemical heterogeneity at its surface and the likelihood of redox reactions and electrolysis phenomena in the water. Previous studies on the springs
and river have shown that their chemical composition was relatively stable over the course of a day
(van Hinsberg et al. 2010; Palmer et al. 2011).
Hence, the springs and river were considered as
good references, due to the constant water flow
and their lower concentration in total dissolved elements in comparison to the lake (van Hinsberg et al.
2010; Palmer et al. 2011). Self-potential measurements were complemented by ground temperature
surveys to detect any thermal anomalies.
Multi-scale wavelet tomography (MWT)
MWT is a signal analysis method using several
wavelets on the SP profiles to investigate depths
of shallow groundwater systems. The wavelets are
based on the dilation and covariance properties of
the Poisson kernel, which is used to analyse potential field signals (i.e. electrical SP, gravity; magnetic) (Moreau et al. 1997, 1999; Martelet et al.
2001; Sailhac & Marquis 2001; Saracco et al.
2004, 2007).
These wavelets are the second and third vertical
derivative (V2 and V3, respectively) and second and
third horizontal derivative (H2 and H3, respectively). Each analysis was made with each wavelet
over 500 dilations for a range of dilation from 1 to
20. In order to avoid artefacts, we only consider
depths found to be significant by at least three of
the four wavelet analyses.
The main component of the electrical source
is the electrokinetic effect, which is associated
with the water flow displacement. Often, the strongest flow displacement is associated with the limit
between the saturated and unsaturated zone (Mauri
et al. 2010, 2012). Hence, this method helps investigate the depths of shallow groundwater systems.
A complete description of the MWT method can
be found in Mauri et al. (2011) with detailed examples found in Mauri et al. (2010, 2012).
Results
Lake temperature
The lake temperature is generally between c. 308C
and c. 458C (Caudron et al. 2015a), but differs significantly in two areas. In the first area, in the NE part
of the lake near the shore, the surface temperatures
are 208C lower (Figs 1 & 2a–c) due to cold water
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KAWAH IJEN HYDROGEOLOGY
Fig. 2. Thermal infrared images of the NE part of the lake (a–c) and the corresponding image in the visual
spectrum (d). Colour bars in panels a, b and c display the temperature measured by the infrared camera. The brown
precipitate on panel d marks an input of cold water.
flowing in from nearby Merapi volcano (a namesake
of the well-known Mount Merapi in Central Java,
Fig. 1). The colder temperatures are also manifested by discoloration (brown precipitate) (Fig. 2d)
and a higher pH (pH 3) than elsewhere in the
lake (pH 0). In the second area, close to the silicic
dome (number 5 on Fig. 1), the lake temperatures
are 1–28C higher, probably due to numerous subaqueous fumaroles (see Lake degassing section).
Sudden temperature drops between October
2010 and January 2011 are related to heavy precipitation (Fig. 3a, b). When the sensor reaches a depth
of c. 3 m below the lake surface, the rain no longer
influences the lake temperature (i.e. after midJanuary 2011, Fig. 3a, b). In contrast, temperatures
measured at the surface by CVGHM observers are
systematically biased during the rainy season. We
also note the sudden increase in early May 2011
(Fig. 3b) that most likely reflects a local anomaly
and/or inefficient mixing of lake waters.
Temperatures of the Banyu Pahit springs
The Banyu Pahit River is a confluence of three sets
of springs (Delmelle & Bernard 2000; van Hinsberg
et al. 2010; Palmer et al. 2011). The first set of
springs (number 1, Fig. 1), which lies just above a
gypsum waterfall, near the dam, has temperatures
of 36.5 –388C (Fig. 3c, d). Sudden temperature
drops were recorded during periods of heavy precipitation (Fig. 3c, d). Long-term temperature variations still generally match those recorded in the
lake (Fig. 3b– d), although these fluctuations are
reduced by the thermal buffering effect of the
rocks through which the water flows (Fig. 3b–d).
Indeed, based on geochemical analyses, the first
set of springs has been suggested to originate from
the continuous seepage of lake waters through the
rock basement (Delmelle et al. 2000).
The second set of springs (number 2, Fig. 1) has
a very similar temperature pattern to the first one,
and the sensor was consequently removed after a
week-long measurement.
The third set of springs, located c. 1 km further to
the west of the first set (number 3, Fig. 1), displays
a clear seasonal temperature variation (Fig. 3e),
with a decrease during the rainy season (November
to May) and an increase during the dry season
(Fig. 3e). In contrast to the first set of springs and
the lake (Fig. 3b–d), temperature measurements
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C. CAUDRON ET AL.
Fig. 3. Results from the continuous monitoring of (a) rainfall (mm) measured at Banyuwangi airport; (b) lake
temperature (8C) measured weekly by CVGHM staff at the dam (isolated black squares) and next to the silicic dome
(isolated black circles) and measured hourly by iButton sensors at the dam (linked black circles); (c) temperature
measured in the first set of springs of Banyu Pahit (8C) (number 1 in Fig. 1); (d) temperature measured in the first
spring of Banyu Pahit (8C) (zoom); (e) temperature measured in the 3rd set of springs of Banyu Pahit (number 3 in
Fig. 1) (8C).
are not directly influenced by short periods of heavy
precipitation, as the sensor is located in a cave and is
therefore protected.
Ground temperature along the crater
rim and in the crater
Ground temperatures were surveyed three times
between 2006 and 2008. Along the crater rim no
thermal anomaly was detected (background temperature of c. 208C). In the crater, only thermal anomalies with very strong gradients (.1508C m21) were
detected on the actively degassing silicic dome.
Lake degassing
Comparison between echo sounding surveys performed in 1996 by Takano et al. (2004) and
in 2010– 11 (this study) show little difference
(Fig. 5a, b) aside from a decrease in lake level
attributed to filling by landslides and/or enhanced
precipitation at the lake bottom (Caudron et al.
2015a). Importantly, numerous sub-lacustrine
fumaroles were detected during our echo sounding
survey (Figs 4 & 5c). The most impressive degassing
plumes are observed in the centre of the lake (Fig. 4)
but, based on visual observations, do not reach the
lake surface. These areas show high Sv values (Fig.
5c) corresponding to high concentrations of matter
in the water column, in this case bubbles and sulphur spherules. Contrary to the Kelud volcanic
lake (Caudron et al. 2012), the presence of sulphur
in the water column prevents deriving gas flux data
directly from Sv values. However, we also find that
degassing occurs close to the active silicic dome
where the bubbles reach the surface (SE, Fig. 5c).
We investigated the concentrations of CO2
within the lake as this gaseous species has proven very useful for monitoring volcanic lakes
(Christenson et al. 2010; Caudron et al. 2012).
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KAWAH IJEN HYDROGEOLOGY
Fig. 4. Echogram of the lake (dotted (red) line on
Fig. 1) and its main features: upwelling bubbles mixed
with sulphur spherules (50 kHz channel). Dark red
colours indicate high concentrations of echoes
(Sv ¼ 230 dB; Sv is the volume backscattering
strength), whereas light blue colours are low
concentrations (Sv ¼ 270 dB). The white line is the
lake bottom. See online version for colours.
The concentrations of gaseous CO2 measured at
12 different locations was all in the range of 5–
14 vol%. Although no spatial anomalies were detected at the lake surface, the concentrations were
higher in 2011 than 2010. This may be explained
by the higher level of volcanic activity (Caudron
et al. 2015b) or by a colder lake temperature
compared to 2010 (Caudron et al. 2015b). Indeed,
CO2 concentration reflects a steady-state balance
between CO2 supplied to the lake by hot springs
(in a dissolved form) and by direct degassing, and
CO2 lost by diffusion at the air –water interface
(Bernard et al. 2013).
Self-potential along the crater rim
Between, 2006 and 2009, the self-potential (SP)
results show only minor variation of the natural electrical potential around the crater and toward the
SW (Fig. 6). Over the years, only two positive SP
anomalies of very limited extent show a correlation
with strong hydrothermal surface alteration (1 and
2 on Fig. 6). Therefore, these two small anomalies
(less than 20 mV in amplitude, a few hundred m in
width) are likely due to the strong surface alteration,
rather than evidence of rising hydrothermal fluids.
In addition, temperature measurements show no
sign of anomalies in either of these locations.
The strongest SP decrease is located on the east
and north rim of the crater (Fig. 6). On the west
side of the crater, increases in the SP values are
associated with decreases in elevation towards the
river spring (Fig. 6). On the south flank of Kawah
Ijen and along the path to and on the main road, the
SP variations are inversely associated with decreasing elevation, which is typical of gravitational
down flow of water in a cold aquifer (Lénat 2007).
Around the crater rim, the analyses of the SP/
elevation gradient show clear inverse gradients
that typically characterize gravitational down flow
(Fig. 7), except for section 4, which shows no gradient at all. Three distinct aquifers can be characterized, the West, the South and the East aquifer. The
latter extends toward the north. Traditionally, as
presented in the work of Lénat (2007), SP/elevation
gradients are best used parallel to the flow direction.
On a crater rim, as on Kawah Ijen, strong topography variations prevent these types of profiles. In
our case, the SP profile (Fig. 7) is perpendicular to
the flow direction, but does not change the interpretation of the inverse SP/elevation gradient as long
as the gradient is clear. No gradient or a normal gradient is often considered to represent rising water
flow (Lénat 2007); however, when the SP profile
is perpendicular to the flow direction, as here, it
would be hazardous to consider section 4 of the profile as a rising flow (Fig. 7). As there is no thermal
anomaly, and because the SP variations are around
20 mV, it is more likely that the SP variations in section 4 (Fig. 7) are due to internal variation in the
aquifer surface geometry.
MWT was used to estimate the depth of the main
sources of electric potential. On average, the aquifers are around 100 m deep (Table 1). Results of
the MWT from SP data consist of 189 depth values
measured with four different wavelets between
2006 and 2009 (Table 1). On the basis of their position along the SP profiles, the depth values were
grouped into 26 depths (8 in 2006, 9 in both 2007,
2008 and 11 in 2009). Using only depths found
with at least three wavelets that are spatially well
located over the four years (along the profile), we
characterize six datasets along the crater rim,
which are named for their relative position from
the crater: North, NE, East, SE, South and SW
(Fig. 8). No obvious change in depth occurs from
2006 to 2009, within the uncertainty of the MWT
analyses (Fig. 8a–f ).
Discussion
Geophysical methods in hyper-acidic
environments
Acidity of the water is an important factor to consider when investigating both surface and groundwater bodies on an active volcano, because (1) it
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C. CAUDRON ET AL.
Fig. 5. (a) Kawah Ijen bathymetry is displayed using contour lines (10 m intervals). (b) Kawah Ijen bathymetry
carried out in August 1996 (from Takano et al. 2004) (c) Sv (mean volume backscattering strength) map. Black dots
represent the echo sounding profiles carried out during two campaigns in 2010 and 2011. Two insets show
echograms acquired by Takano et al. (2004) in 1996 (black and white, upper right) and in 2010 (lower right). Dotted
and dashed lines correspond to the 1996 and 2010 profile locations, respectively.
may affect the physical properties that are measured; and (2) it will attack and corrode equipment.
Previous work on acidity showed that it may
have a significant impact on electrical generation.
Indeed, studies have shown that pH is the expression
of the balance in ions within the water fluid, which
will affect the electrical generation (Guichet &
Zuddas 2003; Hase et al. 2003; Aizawa et al.
2008). In some cases, such as when the pH is c. 3,
it appears that the Zeta potential becomes null and
electrical generation stops. However, it is still not
clear what happens when the pH is c. 0. At Kawah
Ijen volcano, the pH of the water flow (aquifer and
hydrothermal system) approaches 0 and could be
of significant importance for electrical generation.
The spring waters range from pH 1 –6 (Delmelle
& Bernard 2000; Palmer et al. 2011). Our study
shows that SP generation is well established over
time (Fig. 7), but the amplitude of the anomalies
are much lower than on other volcanoes (e.g. Lénat
2007; Aizawa et al. 2008; Mauri et al. 2012).
Thus, in this study while the low pH may affect
the electrical generation, it does not appear to stop
it. Further investigation of Zeta potential is needed
to understand the true effect of hyper-acidic fluids
on electrical generation.
Spring and lake temperatures are difficult to
probe at Kawah Ijen. For in situ monitoring of the
acidic lake waters, a protection similar to that
described above (see Instruments and methodologies section) seems mandatory. Those probes not
directly protected from the acidity were damaged
within a month or less. Thermal infrared cameras
allowed us to detect clear thermal anomalies at a
safe distance. They would be also very useful for
capturing convective cells and/or small explosions
at the surface, such as in the Rincon de la Vieja
and Poás volcanoes (Costa Rica). However, those
instruments are also damaged by the corrosive
gases emitted in the crater (Ramı́rez et al. 2013),
hence, they need to be carefully protected.
Echo sounding techniques proved to be particularly effective for investigating degassing processes
and dynamics in volcanic lakes (Caudron et al.
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KAWAH IJEN HYDROGEOLOGY
Fig. 6. Self-potential maps from 2006 to 2009. Distance and elevation contours, in m. All the data are referenced to
the spring and river, (triangles). Dashed black line is the Banyu Pahit River. Black dots are the SP profiles. 1 and 2
represent hydrothermal areas.
2012), even in extreme conditions such as the
Kawah Ijen environment. However, the presence
of sulphur in the lake waters prevents a straightforward estimation of the CO2 flux, as for Kelud volcano (Caudron et al. 2012). A new approach based
on the impedance contrast might be useful to tackle
this problem and estimate the CO2 flux using echo
sounding methods.
Lake and hydrothermal system structure
and dynamics
A good understanding of the lake and hydrothermal
structure is essential to correctly interpret the
variations in volcanic activity and to improve risk
mitigation. In contrast to its geochemistry (Takano
et al. 2004), the lake degassing and thermal features
are not homogeneous, with clear anomalies controlled by the different aquifers in specific areas.
In the centre of the lake, the absence of a thermal
anomaly at the surface was unexpected considering
the substantial degassing evidenced by the echo
sounding surveys (Figs 4 & 5). Thermal anomalies
were only observed at the surface close to the lake
shores on the eastern side. Although they could
not be echo sounded due to shallow depths, many
areas of small bubbles are observed in those locations, as is the case at other lakes (e.g. Kelud,
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C. CAUDRON ET AL.
Fig. 7. (a) Ground temperature; (b) SP profiles; (c) SP/elevation gradient of the crater rim. The numbers (1– 5)
represent the section of different SP/elevation gradients.
Central Java, Indonesia; Caudron et al. 2012).
A weakened hydrostatic pressure might explain
the preferential gas release close to the shores.
Hence, the lake morphology is a critical factor in
correctly assessing the degassing and thermal processes acting in volcanic lakes.
The four years of SP and ground temperature
measurements did not reveal any significant sign
of shallow hydrothermal activity outside the crater.
The main hydrothermal activity is thus most likely
restricted to the active crater. The hydrogeological
system consists of three aquifers.
The East aquifer flows from Merapi volcano
(Fig. 1) down the slope and is discharged into the
acidic lake (Fig. 6, solid (blue) arrows in Fig. 9).
This is consistent with the persistent cold plumes
imaged using the thermal infrared camera at
different periods of the year. This aquifer is most
likely the main source of groundwater for the acidic
lake.
The West aquifer is clearly visible on each SP
profile and manifests at the surface through several
acidic water springs (Fig. 6, dashed (green) arrow
in Fig. 9).
The South aquifer flows from the south peak of
the crater rim towards the south at least to the caldera floor (solid (blue) arrows in Fig. 9). Based on
the electrical structure, the lack of anomalous
ground temperature and the potable water spring
located downstream, the South aquifer seems disconnected from the acidic lake and the crater hydrothermal system. Between Pondok and Paltuding
(Fig. 9), the fresh water spring (number 6, Fig. 1)
is likely one output of this aquifer based on the
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KAWAH IJEN HYDROGEOLOGY
Fig. 8. Water depth variation between 2006 and 2009. Depths are calculated by MWT based on SP profiles. Only
depths obtained with 3 –4 wavelets are presented here. Error bars reflect the scattering of the data.
topography and SP/elevation gradient that suggests
water flow toward the spring (Fig. 6c).
An ephemeral North aquifer may be present
inside the north rim of the crater, but the source
of its water is not clear (Fig. 6). Information from
a Dutch colonial topographic map reports the existence of an ephemeral aquifer (in Van Hinsberg
2000), which was believed to exist due to the presence of evaporite minerals and an observed spring;
no spring has been observed recently. Based on
the topographic surface and the SP/elevation gradient from 2006–8 on the north rim, the surface water
flows from east to west at a similar or shallower
depth than the north rim water table (Fig. 8a, for
North aquifer, Fig. 8b, c, for East aquifer). Therefore, we may surmise that the North aquifer is only
supplied with water coming from the East aquifer.
Such water flow is likely to occur only when the
East aquifer level is very high.
Outside the crater area, the only hydrothermal
expressions are of small extent and located SE and
SW along the upper part of the south flank of the
volcano (Fig. 6). Hydrothermal activity is not supported by ground temperature anomalies. On the
SW flank, outcrops reveal no sign of hydrothermal
alteration of the rocks. On the SE, numerous outcrops show a high level of hydrothermally altered
rock. However, we could not determine whether
these deposits are the remnant of former hydrothermal activity in this area or if they consist of
remobilized deposits from a past eruption. The
fact that the SE and SW hydrothermal expressions
of zone 1 (Fig. 6) are of small extent and not associated to any ground temperature suggests that the
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C. CAUDRON ET AL.
Table 1. Water depths
Profile
name
North
North-East
East
South-East
South
South-West
Years Number Distance
of
x (m)
wavelet
sx
(m)
2006
2007
2008
2009
2006
2007
2008
2009
2006
2007
2008
2009
2006
2007
2008
2009
2006
2007
2008
2009
2006
2007
2008
2009
20
10
17
13
8
39
6
29
25
26
7
6
50
19
14
13
54
40
14
5
10
12
5
6
4
3
3
3
4
4
3
4
4
3
4
3
3
3
4
3
3
4
4
4
4
4
4
4
3118
3136
3236
3192
2605
2582
2550
2761
2103
2144
2161
2064
1893
1801
1839
1934
1233
1234
1144
1106
390
454
608
472
Depth
(m)
288
2104
2146
251
252
2145
2113
256
2120
2146
293
269
238
2344
2113
245
240
2137
256
289
2131
268
242
292
sZ
(m)
33
12
58
21
19
51
18
26
41
26
38
10
24
117
43
14
16
37
18
42
60
30
19
60
Elevation
(m above
sea-level)
2253
2223
2200
2269
2340
2233
2288
2315
2281
2238
2312
2305
2346
2007
2257
2315
2305
2195
2290
2250
2164
2232
2317
2180
UTM in m
East
North
196 088 9 108 861
196 086 9 108 857
196 055 9 108 892
196 081 9 108 898
196 515 1 908 642
196 553 9 108 603
196 585 9 108 564
196 427 9 108 698
196 654 9 108 200
196 660
918 200
196 663 9 108 227
196 669 9 108 106
196 661 9 108 002
196 625 9 107 892
196 642 9 107 896
196 659 9 107 987
196 241 91 077 583
196 216 9 107 577
196 114 9 107 616
196 070 9 107 643
195 612 9 108 015
195 637 9 107 934
195 692 9 107 858
195 635 9 107 951
Topography
2341
2327
2346
2320
2392
2378
2401
2384
2374
2384
2405
2374
2384
2351
2370
2361
2345
2232
2346
2339
2295
2300
2359
2272
Water depths calculated by multi-scale wavelet tomography (MWT) using the SP profiles acquired on the crater rim (s characterizes the
scattering of the data). UTM, universal transverse mercator.
hydrothermal system was reduced due to a decrease
of activity in this part of the volcano. Alternatively,
the important formation of hydrothermal deposits
(Scher et al. 2013; Berlo et al. 2014) may have selfsealed the hydrothermal fluid pathway somewhere
between the surface on the SE flank and the crater
hydrothermal system.
Perspectives for continuous monitoring
In active craters, lakes are constituent parts of the
hydrothermal system and, as such, record changes
occurring at depth (Rowe et al. 1992; Ohba et al.
1994; Terada et al. 2011). Surrounding hydrogeology and lake volume are important factors that may
buffer the changes occurring in the deeper parts of
the hydrothermal systems. Changes in volcanic
lake systems are essential to track as acidic lakes
are considered to be constituent parts of a hydrothermal system. Therefore, precise observations and
analyses of hot volcanic lakes may reveal even
slight changes in the input of volcanic fluids originating from the underlying hydrothermal system
(Terada et al. 2011).
Monitoring a spring related to the hydrothermal
system allows for acquiring information safely,
especially during volcanic crises. Moreover, this
may be particularly useful in Kawah Ijen where
the important volume of the lake buffers the temperature fluctuations. However, the springs should
not be too diluted by meteoric waters. According
to Palmer (2011), the third set of springs (number
3, Fig. 1) may represent the deep hydrothermal system and could thus provide evidence for an
increase in volcanic activity before any change in
the volcanic lake. Our temperature data do not support the hypothesis of a deep hydrothermal source
derived from the water geochemistry (Palmer
et al. 2011), but rather indicate that the spring
water is buffered by cold water input near the surface. Therefore, the third set of springs does not
appear to be connected to the lake, but rather connected near the surface to meteoric waters coming
from another valley unconnected to the crater of
Kawah Ijen. Hence, our results suggest that this
spring set is of limited benefit for future temperature monitoring. Closer to the crater, the first set
of springs (number 2, Fig. 1) mostly derives from
lake seepage. The strong buffering effect makes it
less interesting from a monitoring perspective
than the lake itself, currently monitored every
hour by iButton sensors.
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KAWAH IJEN HYDROGEOLOGY
Fig. 9. Summary of the water flow structures on the Kawah Ijen volcano. (a) Digital elevation model (derived
from SRTM (Shuttle Radar Topography Mission) data) merged with the 1:20 000 scale map from the topographical
survey (1918; van Hinsberg et al. 2010), coordinates are in UTM; (b) 2D view and cross-sections (D –C: north
and A –B: east). Solid (blue) arrows indicate fresh water flow, dashed (green) arrows acidic water flow from the lake,
dotted (orange) arrows the fluid from deep hydrothermal systems, dark solid (red) circles the underwater degassing
locations, pale shaded (yellow) areas the old silicic dome, darker shaded (red) areas the active silicic dome, pale
solid (blue) circles the set of springs, pale (blue) line the lake surface, diamonds are aquifer depths (derived from
MWT (multi-scale wavelet tomography)) and small black dots indicate SP profiles. See online version for colours.
Apart from the dam area, it would be useful to
install a probe in the centre of the lake and another
in the waters near the active silicic dome. The centre
of the lake is less affected by streams of meteoric
water flowing from the crater slopes. The fluids
near the dome are less buffered than elsewhere
due to a lower water level and interestingly, lie
above a thermal and degassing anomaly. Hence, it
would be useful to install a probe in these locations.
From a technical point of view, equipping the centre
of the lake is particularly difficult to achieve as it
requires the use of a buoy.
While echo sounding may be very interesting
for monitoring, it requires the use of a boat on the
largest acidic lake in the world. Alternatives exist,
however, such as the use of a remote-controlled
boat. A single traverse every month that crosses
the main degassing foci (Fig. 5) could be controlled
safely from the lake shore. A hydrophone would
be also very appropriate for Kawah Ijen. This instrument successfully detected hydroacoustic noise
precursors before the 1990 eruption in Kelud volcanic lake (Vandemeulebrouck et al. 2000), and
was installed in a very similar lake environment
(Ruapehu, New Zealand, Hurst & Vandemeulebrouck 1996).
Another parameter that would be very interesting to routinely monitor in Kawah Ijen is the CO2
concentration in the lake or above the lake surface.
Previous work on CO2 monitoring gives useful
information on the magmatic source and on the
magmatic dynamics in Kawah Ijen (VigourouxCaillibot 2011; Scher et al. 2013). This gas is
a relatively inert, pressure-transmitting medium in
the hydrothermal environment (Christenson et al.
2010). In other volcanic lake systems, such as the
Kelud system before the 2007 eruption, it was the
earliest sign of an impending volcanic eruption
(Caudron et al. 2012).
The use of geophysical instruments was very
successful from a non-continuous perspective, but
constitutes a significant challenge for continuous
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C. CAUDRON ET AL.
monitoring in an environment such as Kawah Ijen.
From the MWT results, the East, South and North
aquifers are at about 100 m below the topographic
surface. Along the crater rim, each of these aquifers
is slightly above the lake level (Fig. 9). The uncertainty on the MWT results (mostly between 10 and
60 m) does not allow us to characterize any clear
variation over time of the depth of the top of the
aquifers. It should be noted that in 2007 (Fig. 8), 4
of the 6 inferred water table elevations showed
a decrease of 30 m or more (depth name: NE, E,
SE, S, Table 1). Along the crater rim, this decrease
in the water table elevation correlates with the 2007
El Niño drought. This study cannot prove that there
is a correlation, as there is no rainfall record for
Kawah Ijen. However, it would be interesting in
the future to investigate possible correlations
between drought and depth of the aquifers and possible implications for the hydrothermal system of
Kawah Ijen. As no shallow fluid is infiltrated in
the SE and SW hydrothermal areas, a future increase
of the electrical signature could indicate an infiltration from the deepest parts toward shallower areas.
New infiltration from the self-sealed hydrothermal
system within the upper part of the edifice could
locally increase the pore pressure within the rock
and, hence, generate rock fracturing that could contribute to destabilization of the volcanic edifice.
This study highlights the existence of a selfsealed hydrothermal system that has important
implications for monitoring Kawah Ijen. Using
modelling approaches coupled to observations,
Christenson et al. (2010) concluded that the 2007
Ruapehu volcano (New Zealand) eruption was, at
least initially, a gas-driven (effectively phreatic)
event originating from an initially sealed hydrothermal system. Hydrothermal seals allow for development of pressurized, vapour-static gas columns
beneath a vent (Christenson et al. 2010). Their failure led to decompression of the region beneath the
seal, resulting in a downward-migrating pressure
transient, and an upward expulsion of material in
the rapidly expanding fluid phase. During the recent
2011–2012 (Caudron et al. 2015b) and 2013 unrest
at Kawah Ijen, several seismic analyses and lake
parameters (sudden seismic velocity drops, temperature increases preceded by drops, level increases,
upwelling bubbles and increased gas emissions)
indicated substantial build-up of pressure in the
shallow system which could be attributed to an efficient sealing of the hydrothermal system followed
by a sudden release of stored fluids.
Conclusions
using geophysical methodologies providing new
insights into one of the most exotic, but dangerous,
magmatic– hydrothermal systems on Earth.
No significant hydrothermal activity is found
outside the crater of Kawah Ijen volcano. Longterm and intense hydrothermal alteration, as well
as possible preferential fracturing, seems to have
allowed the intense shallow hydrothermal alteration
system to completely seal itself within the upper
volcanic edifice. This would leave the crater as the
only path to release the volcanic fluids. Therefore,
our results confirm past work on the geochemical
signature of the hydrogeological fluids and volcanic
gas (Palmer et al. 2011; Vigouroux et al. 2012;
Scher et al. 2013; Berlo et al. 2014). While the
southern aquifer is completely isolated from the
lake and flows SE from the volcano, the eastern
part of the lake receives a high amount of cold water
flowing from nearby Merapi volcano. The North
aquifer shows signs of fluctuations in its electrical
potential signature, which are currently not fully
constrained. The East, North and South aquifers
appear to have their water table at an elevation that
is above the lake level, but between 60 and 100 m
below the crater rim. An acidic aquifer discharges
on the west and feeds the Banyu Pahit River.
Thermal and gas anomalies are found in the
SE part of the lake near the active silicic dome.
Although the main conduit discharges an impressive
amount of gas in the centre of the lake, no temperature anomaly was observed at the surface. Lake
shores are the loci of bubblings associated with
upwelling of hot waters, probably due to a weaker
hydrostatic pressure and less efficient convection.
The lake morphology ultimately also dictates the
degassing and thermal pattern.
The self-sealing also has strong implications for
monitoring Kawah Ijen. Hydrothermal seals allow
for development of pressurized, vapour-static gas
columns beneath a vent (Christenson et al. 2010).
Although it could only be suggested during the
last sequence of unrest from 2011–13, several indicators suggests a substantial build-up of pressure in
the shallow system followed by a sudden release of
the stored fluids. We surmise that this process
should be closely investigated in the future using
modelling approaches coupled to available monitored parameters and observations.
Results and analyses from different geophysical surveys allow us to better understand how
the hydrothermal system is structured. They also
improve our knowledge of the lake dynamics and
morphology, which is essential to correctly interpret
the nature of the lake variations and better forecast
catastrophic events.
After more than a century of chemical investigations, the Kawah Ijen volcano has been surveyed
We are grateful to CVGHM support on the field and in
Bandung, and particularly to the observers of Kawah
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KAWAH IJEN HYDROGEOLOGY
Ijen, Pak Heri and Pak Parjan, Pak Im, Jumanto, but also to
the students (Antoine Triantafyllou, Zack Spica, Julien
Brack, Sarane Sterckx, Raphael De Plaen, Julie Oppenheimer) and numerous volunteers who greatly helped in the
field. This work is partly funded by a Belgian federal science policy Action 2 grant (WI/33/J02) and a Canadian
NSERC-CRD grant to G. Williams-Jones.
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